As the application is mostly complete you really do not require full learning/understanding to customize the cart. Unless you are thinking of writing your own.
Web Design is a broad field that includes many things - but my base definition is that there are artists that can create beatifully designed page art/layout - then there is a web designer that converts this art into a workable web page. Normally these are different individuals(something to do with the sides of the brain thing). People who think logically or structured normally are not very adept at the art side of design and the opposite is normally true also.
So which are you?
Online for most things basic: html, css, php
http://w3schools.com/
http://www.htmldog.com/
http://www.htmlgoodies.com/
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